1. Field of the Invention
The present invention is related to a communication network including at least one transmitter having a speech encoder for deriving from an input signal a plurality of prediction coefficients of a prediction filter and an excitation signal, and a multiplexer for deriving a symbol sequence including a representation of the plurality of prediction coefficients and a representation of the excitation signal, the communication network being arranged for transmitting the symbol sequence to a receiver with a speech decoder. The present invention is also related to a receiver, a decoder and a transmission method.
2. Description of the Related Art
A communication network according to the preamble is known from the article "Real Time Communication on the Net" in PC Magazine, Oct. 8, 1996, pp. 102-120.
With the rapidly growing number of users of the Internet and the advanced developments in speech coding, Internet based telephony may become a real alternative for the common PSTN network. This is in particular true for international calls, due to the substantial savings that can be obtained. A communication system according to the preamble uses a speech coder in order to reduce the required bitrate from the standard 64 kb/s to a substantially lower rate ranging from 2-16 kb/s.
Most speech encoders derive parameters for a synthesis filter in the speech decoder together with a suitable excitation signal to be applied to the synthesis filter. The determination of the filter parameters is based on so called prediction parameters which define the short time spectrum of the input speech signal. The excitation signal can be found in different ways. One way, the analysis by synthesis method, involves synthesizing a speech signal using a predetermined number of excitation signals, and selecting the excitation signal which results in a synthetic speech signal which gives the best match with the input speech signal. Another way to find the excitation signal is to filter the input speech signal with an inverse synthesis filter, and encode the residual signal at the output of said filter.
A problem with the communication system according to the preamble is that it is possible that more speech traffic is offered to the network than it can handle. This can lead to loss of a connection or to a large delay of the speech signal to be transmitted. It is also possible that a speech signal is encoded according to a format which is too complex to decode with the decoder available at the receiver.